As a representative of a third-generation semiconductor material, gallium nitride (GaN) has many excellent characteristics, for example, a high critical breakdown electric field, a high electron mobility ratio, a high two-dimensional electron gas concentration and a good high temperature working ability etc. A third-generation semiconductor device based on the gallium nitride, such as a high electron mobility transistor (HEMT) and a heterojunction field effect transistor (HFET) etc., has been put into applications, and has obvious advantages especially in the fields such as radio frequency and microwave etc., which require high power and high frequency.
In order to increase the two-dimensional electron gas concentration of a gallium nitride HEMT device, hence obtain a higher power of the device, the conventional method is to use an AlGaN/GaN-based HEMT having a high Al composition. However, increasing the composition of Al causes an AlGaN film to be under more tensile stress, if the tensile stress exceeds a certain extent, microcracks may occur in the AlGaN layer, resulting in a yield rate problem or a reliability problem of the HEMT device.